THE PIONEER ANOMALY ----------------- John Pazmino NYSkies firstname.lastname@example.org 2007 April 15
Introduction ---------- Each year in March, the American museum of Natural History convenes the Isaac Asimov memorial debate. This is a panel discussion of a specified topic in astronomy among several experts in the profession. On 2007 March 26, the topic for the debate was the 'Pioneer anomaly'. Because this subject involves both astronomy and space travel, the audience had a dozen or so delegates from NYSkies and National Space Society. Neil Tyson, Hayden Planetarium director, was the panel moderator. Panelists for this debate were: John Anderson, JPL Edward Belbruno, Princeton U Gary Kinsella, JPL Irwin Shapiro, H-S CfA Slava Turyshev, JPL Anderson is generally credited with first recognizing and announcing the anomaly. He and Turyshev are still actively investigating it. Belbruno is an astrodynamicist specializing in interplanetary trajectories. Kinsella is an aerospace engineer working with spacecraft systems. Shapiro is an astrophysicist and a skeptic of alternative solutions beyond current physics. The audience, packed into the the Museum's LeFrak Theater and overflowing into its Kaufmann Theater, numbered about a full one thousand! This is a stunning testament to the vivid science culture prevailing in New York City. Due to a generally low-level description and commentary about the Pioneer anomaly, there grew up around it a body of pseudoscience and faux-fact that at times inhibited a mature investigation of it. The public may hear of the Pioneer anomaly wrapped in dialog about abnormal science or even nonscience.
The Pioneer missions ------------------ Pioneer-10 and Pioneer-11 were the first two spacecraft to leave the solar system, following their exploration of Jupiter and Saturn. Pioneer-10 launched on 1972 March 2; Pioneer-11, 1973 April 5. Altho their missions were long ago done, the craft continued to collect space environment data and were tracked routinely thru NASA's Deep Space Network as probes in the far outer realm of the solar system. Pioneer-11 signals broke off in October 1990. Pioneer-10 may still be transmitting but by March 2006 its signal grew too weak to discern against the general background radio noise. Following the planet swingbys both craft entered a hyperbolic trajectory that carries them almost linearly away from the Sun while staying close to the ecliptic plane. By chance, due to their separate mission profiles, Pioneer-10 is heading toward western Taurus; Pioneer-11, central Scutum. By 2007 Pioneer-10 is about 92AU from the Sun; Pioneer-11, 73AU. Both craft were simple, compared to today's payloads, and had only a few instruments onboard. They were powered by radioisotope generators because at the distance of Jupiter and beyond solar panels would collect too little energy. Radioactive power sources were used in future deep space missions, like Galileo, Cassini, New Horizons.
The anomaly --------- As the two Pioneers receded from the Sun, their trajectories were affected by fewer and fewer natural forces. Notable was the ebbing of solar radiation pressure that imparts a slight outward force. By 1980m when the ships were well away from other planets or substantial mass, their paths were governed essentially only by the gravity field of the Sun. As other influences waned, there appeared in the tracking data a small acceleration toward the Sun. This is found in the Doppler shift of received frequency of the Pioneer rad io transmissions. Only a radial component was ever monitored in the outer solar system after the main part of the probe's mission was complete. There is substantially no transverse, or proper, motion on the celestial sphere because the spaceships are travelling pretty much straight away from Sun and Earth. This excess acceleration IS tiny, about 8.7e-10 m/s2. Or 8.7 ANGSTROMS per square-second!! If this were imparted to a cart at rest, how long would it take for the cart to get up to a brisk walking pace of 4m/s? About 158-1/2 years! The ability to even detect such a minuscule acceleration comes from the extreme maturity of processing radio and electronic data. In fact, the excess frequency shift associated with this acceleration is only 6e-9Hz/s in the S-band. The effect, now popularly called the 'Pioneer anomaly', was first recognized by 1980 but first formally investigated in 1998. Accounts about the anomaly can be confusing. The effect is ekked out of data from about 1980. It was examined on and off since then thru the 1990s. It was first specificly noticed to the world in 1998. Thus, there arose a 'coverup' mentality among some space fans. NASA knew about the discordant motion and held it secret for over 15 years?! One of the major peculiarities of the Pioneer effect is that soonest the craft entered the exit path out of the solar system, the magnitude stayed constant over both distance and duration thereafter.
Other missions ------------ We now have four probes in the far edge of the solar system. The two Pioneers, Voyager-1, and Voyager-2. The Pioneer effect is not well observed in the Voyager probes because they routinely alter their speed and orientation by firing jets. The two Pioneers are spin- stabilized so their radio antennae aim toward the Earth. They use no jets since their planet bypasses. When the various missions to Pluto were planned in the late 1990s, one function was to investigate the Pioneer anomaly with dedicated instruments. However, the only Pluto mission commissioned was New Horizons, launched in January 2006. It took one swingby of Jupiter in February 2007 and is now on a recession path from the Sun. New Horizons can not be used for Pioneer effect examination because of the intermittent and complex radio system it carries. With nothing to do until it reaches Pluto in 2015, New Horizons is hiberating for almost the entire journey from the Jupiter flyby until then. Once every few months it automaticly wakes up to send a short message of vital measurements back to Earth. After these are acknowledged and OKed by Earth, the craft returns to sleep. Other missions, like Galileo, Cassini, Ulysses, suggest the anomaly affects them, too. Their trajectories and comms are too varied and complex for a 'clean' confirmation. This leaves ONLY the two Pioneer data archives for future inquiry into the Pioneer anomaly.
Acceleration ---------- In vernacular speak, an 'acceleration' is a speedup of motion. To describe a slowdown of motion, the word is 'deceleration'. In physics only the term 'acceleration' is used for both changes of speed. The slowdown of speed is handled by a negative value of acceleration. One reason to have only the one concept is that acceleration is a vector. Unless there are deliberate conditions that confine motion to a linear path, like a train on its tracks, it can not be assumed that a deceleration is opposite to an acceleration. On a spaceship, the push, by firing the rockets, can be in any direction relative to the forward motion. The idea of a unique sense of deceleration loosens up. In the Pioneer cases, the craft are moving almost radially away from the Sun and are suffering a slowdown or deceleration. They are not going as fast as they are calculated to move. Yet we speak of the anomaly as an excess acceleration, which can be taken for a speeding up beyond the calculated motion. This jargon can confuse some listeners at first.
Vectors ----- Acceleration is a vector, like velocity. To completely specify acceleration a direction must be included in addition to its magnitude. In the usual popular discussions, this feature of the pioneer anomaly is commonly missed out. In fact, the value of the excess acceleration is prevalently cited with no signum, neither plus nor minus. Because the monitoring of the excess is done solely by radial velocity measurements from the received frequency of the Pioneer signals, only the line-of-sight component is known. There could be a component in the tangential plane but this is for the Pioneer craft not sensible. The trajectory is so nearly radially from the Sun that there is no proper motion in the tangent plane to work out a 3D model of the anomaly. Because the value is about the same for both Pioneers and is arguibly also the same for other spacecraft, the feeling is that the entire vector is more or less aimed back to earth. Assuming that the outward direction is positive, the anomaly carries the negative signum. For a reverse signum convention, the anomaly is positive. However, the direction of the vector is uncertain and could be one of at least four according as the hypothesized cause. It could be aiming at the Sun, aiming at the Earth, aligned with the flightpath, aligned with the craft spin axis. It turns out that all four are close within a couple degrees due to the remoteness of the craft from Sun and Earth, the deliberate pointing of the spin axis toward Earth, and the linearly outward track of the ship.
Missing records ------------- One of the disgusting realities of human endeavors is the haphazard way we preserve our history an legacy. This is specially prevalent with so much of human information collected and stored on fragile media and read by proprietary devices. Both have short lifetimes, of a decade or so! Paper records are not exempt from short life. Old books and blueprints are often discarded or lost after their use is deemed over and done. In particular, working papers, often with crucial annotations and side calcs, are routinely tossed after the final formal report for the project is issued. The panel brought out two instances associated with the Pioneer anomaly. One potential source for the acceleration is thermal recoil force. The internal parts of the spacecraft produce heat radiation. When this is released from the outer surface of the ship, a tiny recoil force is produced. To study this effect, a complete and accurate description of the craft is required. It turns out that JPL did lose, by misplacement or rearrangement of offices, many of the prime paper records for the Pioneers. It become a 'treasure hunt' to find these paper and speak with project workers. The thermal specification of the Pioneers is still incomplete but encouraging. About 70% of the anomaly can be traced to thrust-producing thermal radiation. The leftover 30% could be found in the missing records.
Ancient computer -------------- The radio reception from the Pioneers was stored on magnetic tapes, of the old 9-track kind so prevalent in mainframe computer centers of the 1960s. The computers that read these tapes are by the 1980s almost entirely replaced by other models that did not use these tapes. Hence, the material was orphaned, so to speak, with no way to even look at them. This situation is prevalent thruout the space program and even other large-scale government and corporate operations. As computers are replaced, only certain data, often in a half-hearted way, are transferred to new media accommodated by the new machines. The result is that orphaned information ends up abandoned and eventually discarded. On the home computer scale, this is happened twice in the last ten years. First, the former 120mm 'floppy' discs were replaced by the 90mm 'stiffy' discs in the 1980s. New home computers no longer had the floppy discdrive, rendering all the old floppies useless. Today, the 90mm discs are phasing out in favor of CDs, with current computers having no stiffy discdrive or only an accessory one. In addition to the media problem, the issue of actually getting intelligence from the 0/1 pattern remains. The paper instructions telling how the information is written and what the encoded values mean may be lost or otherwise not to hand. So it is not at all a matter of simply ordering -- and funding -- a study of the Pioneer anomaly.
Defying physics? -------------- One very common feature of a discussion of the Pioneer effect is its disobedience of physics. This notion leads to weird ideas of forces outside present science, and into the field of parascience and nonscience. There is a distinction to be emphasized here. One is that there is an unknown, but ultimately discoverable, gravity force. This force, from some distribution of ordinary mass, produces the observed extra acceleration. This mass can be concentrated in a few bodies or diffused in a gas or dust region. It could be in the 'dark matter' permeating the Milky Way. The other is that there is an unknown, but discoverable, nongravity force out there. Gravity is only ONE kind of force; there are hundreds, if not thousands, of other influences that do not arise from gravity. The thermal thrust idea is but one. An other may be thrust-producing leak from the residual fuel. The latter would be unknown because we can not examine the craft close up and there are no leak-monitoring instruments on it.
Two examples ---------- Tyson, in his role as animateur for the debate, put out two examples where solar system bodies with anomalous trajectories led to an ordinary and extra ordinary solution. The deviation of Uranus from its gravitational path, as calculated from the forces of the other known planets, eventually yielded the new planet Neptune. There was no 'new physics' but a smashing victory for the existing science. The deviation of Mercury's motion failed to turn up any new bodies, provisionally named Vulcan. It generated an allnew science, the Einstein relativity theory. Which way will the Pioneer anomaly take us?
Curious calculation ----------------- Dr Belbruno offered a curious calculation. He worked out the effect of the Milky Way on the solar system. Using reasonable values for the mass of the galaxy and our radius from its center, he got, rounded, 8e-10m/s2! This is precisa mente the value of the Pioneer effect!! He emphasized this was a rough calc, only for the magnitude, and it is the gravity field strength at the solar system. It holds the solar system in galactic orbit. He also didn't know off hand the vector direction of the Pioneer ships from the galactic center. He cited 400 billion suns mass and 27,000 lightyear radius. From these and the usual formula for the gravity field strength, I get
GFS = gamma*(400 billion suns)/(27,000ly)^2 = (6.67e-11n.m2/Kg2)*((400e9)*(1.99e30Kg/sun)) /((27,000ly)*(9.46e15m/ly))^2 = 8.14e-10m/s2
Belbruno's math checks. His mass of about 400 billion suns is a recognition of the dark matter within the galaxy. The banter from the panel indicated that what is really needed is the differential force on the Pioneer, due to its slightly farther radius from the galactic center. By luck it is heading about in opposition to the galactic center. The easiest ay to get this is to take the derivative of the GFS formula, to obtain the change in strength per meter of radius.
1der(GFS, radius) = (-2)*gamma*(400 nillion suns)/(27,000ly)^3 = (-2)*(6.67e-11n.m2/Kg2) *((400e9)*(1.99e30Kg/sun)) /((27,000ly)*(9.46e15m/ly))^3 = (-6.40e-30m/s2)/m
For the extra radius out for Pioneer, 92AU, I get
diffGFS = (-6.40e-30m/s2)/m*(92AU)*(1.49e11m) = -8.77e-17m/s2
This is the differential, or tidal, acceleration of Pioneer relative to Sun due to tide action of the Milky Way. This is too low by a factor of ten million. More than that, as a tidal acceleration, it tends to move the spacecraft FASTER away from the Sun, not slower. Or, the Sun falls a bit faster than Pioneer, leaving the ship behind, farther away than if they fell at the same rate. A similar reasoning applies if Pioneer were heading toward the galactic center; it falls faster than SUn, pulling away from it. The above discourse derives from the dialog at the lecture. In the days following the lecture, Dr Belbruno pointed out to me that the impression offered above is misleading. In the brief moment of his comments he missed out details of a larger idea he holds. The acceleration of 8e-10m/s2 is not just a coincidence, but is the actual value; the galaxy itself is partly the CAUSE of the Pioneer anomaly! What is belbruno's idea? He didn't get the chance to lay it out at the debate. No one else on the panel caught on to it. He didn't elaborate it in his later comms with me. He will present it at the 'New trends in astrodynamics' conference on 27-29 June 2007 at Princeton University. Information about this conference is at 'www.spaceroutes.com/astrocon'. End of June is only 3 months from end of March, when the Asimov debate took place. It's a long 3 months to wait; wait we must.
Overall consent ------------- None of the panelists argued for a new branch of physics. All allowed that the Pioneer effect seems to be solvible within present science. It will be increasingly harder to study the problem because there is no hope of gathering new data -- the spaceship are both out of action -- and eventually the data in house will decay or be abandoned. The panel agreed that the existence of the anomaly, while being an intriguing problem of science, will not impede the future exploration of the solar system. It is just too small to louse up spaceship trajectories, specially if the craft has propulsion.
Future work? ---------- Can the anomaly be confirmed by tracking natural objects in the outer solar system like planet Pluto or Kuipeer body? Not really. The effect comes out from comparing the calculated path with the observed one and the observed path must be ultra accurate. So much so, that only, so far, radio or other electronicly processed frequency, is able to bring out the truly minute excess acceleration. Pluto and Kuiper bodies arre observable only in the optical or adjacent IR band and their astrometry is still rather loose. Further more, we then obtain where these objects are with no way to predict where they should be. How about sending out a dedicated new vessel for just probing the Pioneer effect? ESA in 2004 had such a mission on the drawing boards, but it was soon called off. This would be perhaps the best method of attacking the problem, but it still is a $200 million project for the satellite and launch. In today's funding constraint there is no hope of such a mission soon. Rescue of the Pioneer data archive is imperative, not only for the anomaly. In general, NASA has warehouses of old electronic media that it no longer can extract data from and no longer knows what the data are. By good luck, there can be a retired computer that can readout the media and transfer their contents to new ones. In most situations, no such fortune prevails.
The solution ---------- The Asimov panel offered no definite solution, but gave the audience th feeling that the anomaly is a situation under control and accounted for. In the absence of having the Pioneer craft on a lab bench, glitches in it probably will never be uncovered, however ordinary they may be. Even so, it does not explain how the excess acceleration is, as far as the extant records can show, a constant amount regardless of distance, direction, duration for all the spacecraft affected by it. And so, it could be that pioneer-10 an Pioneer-11 will sail into the interstellar abyss, taking their secret with them.